Electrical conductivity drops in the brine

[rudolff16]

Dear Phreeqc members,

I have a question regarding the correlation between the electrical conductivtiy and salinity of a brine sample.
As known the electrical conductivtiy has linear or polynominal correlation to the salinity, but after a threshold of 300 g/l salinity, the electrcal conductivity starts to drop!! are there any explination for that phenomina?

General Analysis (mg/l)
pH 6.9
Ca 54.8
Mg 63712
K  34037
Na 26278
Cl 217861
SO4 51879
HCO3 116
Br    1324

Electrical conductivtiy 153 mS/cm
Salinity                     393,93 g/l

[dlparkhurst]

First, here is a web page that describes the method of calculating specific conductance in PHREEQC based on the diffusion coefficients of ions, https://hydrochemistry.eu/exmpls/sc.

Below is example 17b (in manual and in examples directory of the distributions) with a couple of USER_GRAPHs added. The example evaporates Black Sea water, and indeed once concentrations increase to halite precipitation, the specific conductance decreases. Basically, the concentration of Na is decreasing due to precipitation of halite, while the concentration of Mg+2 increases; total salinity increases slightly. Na+ has nearly double the diffusion coefficient of Mg+2, so the specific conductance decreases.

[dlparkhurst]

I may have been too quick to write the last reply. The diffusion coefficient of Mg+2 is about half of Na, and the concentration of Mg+2 will be half of Na+ for the same positive charge, but the mobility depends on Z^2, which is four times greater for Mg+2. In any event, the calculated specific conductance in the previous example does decrease.